The invention discloses a strain of bacterium lacticum and a ginseng fermentation method for increasing rare saponin. The ginseng fermentation method comprises: inoculating the seed fermentation broth of bacterium lacticum having the preservation number of CCTCC M 2016372 into a ginseng liquid according to an inoculating amount of 1-6%, wherein the fermentation conditions comprise that 33-40 DEG C shaker culture is performed, the rotation speed is 70-160 r/min, and the fermentation time is 18-72 h, and the ginseng liquid is obtained by adding water to ginseng and beating. According to the present invention, the contents of Rh1+Rg2, F1, Rh2, 20(S)-Rg3 and R0 in the ginseng fermentation broth are greatly improved compared with the common bacterium lacticum.